In the Long Term Evolution (LTE) mobile communication system, a User Equipment (UE) in the Evolved Packet System (EPS) Mobility Management Idle (EMM-Idle) status initiates on its own initiative an initial Non-Access Stratum (NAS) procedure to the network side for various purposes of transmitting data, signaling, a location update, etc., to the network side.
If the UE in the EMM-Idle status has user data or signaling in the uplink packet switching domain to be transmitted, then the UE initiates a Service Request procedure in the Packet Switching (PS) domain. Through this procedure, the EMM status of the UE is transferred from the EMM-IDLE status to the EMM-Connected status, and an S1 bearer connection between an Evolved NodeB (eNB) and a Mobility Management Entity (MME) and a radio bearer connection between the UE and the eNB are established. The above UE initiated Service Request procedure may alternatively be triggered upon reception of a paging message from the network side, and the UE will initiate the Service Request procedure by transmitting a Service Request message to the network side in the following cases:
1. the UE in the EMM-Idle status receives a paging request message transmitted from the network side;
2. the UE in the EMM-Idle status needs to transmit uplink user data;
3. the UE in the EMM-Idle status needs to transmit uplink signaling of the LTE; and
4. the UE in the EMM-Idle status needs to transmit uplink signaling of the cdma2000 system.
Furthermore, the UE will initiate the Service Request procedure by transmitting an Extended Service Request message to the network side when Mobile Originated/Mobile Terminated (MO/MT) Circuit Switching (CS) fallback is required for the UE in the following cases:
1. the UE in the EMM-Idle or EMM-Connected status needs to transmit a CS fallback request; and
2. the UE in the EMM-Idle or EMM-Connected status needs to transmit a CS fallback response to a CS fallback request upon reception of the CS fallback request transmitted from the network side.
In addition to the Service Request and the Extended Service Request message transmitted from the UE, the UE in the idle status may also transmit other messages corresponding to the initial NAS procedure including a Tracking Area Update Request message corresponding to a Tracking Area Update (TAU) procedure, a Detach Request message corresponding to a Detach procedure, etc.
A processing flow of a paging procedure in an existing mobile communication system is as follows.
In a paging message from an MME to an eNB, the MME indicates in a CN domain IE indicator whether the paging procedure corresponding to the paging message is triggered for the Circuit Switching (CS) domain or the packet switching domain. The eNB transmits the paging message to a UE, the UE transmits a Service Request message to the MME upon judging from the CN domain IE indicator in the paging message that the paging procedure corresponding to the paging message is triggered for the PS domain, the MME initiates a context setup procedure upon reception of the Service Request to establish an S1 bearer connection between the eNB and the MME and a radio bearer connection between the UE and the eNB, and the UE and the network side can transmit data over the established bearer connections after the bearer connections are established; and the UE transmits an Extended Service Request message to the MME upon judging from the CN domain IE indicator in the paging message that the paging procedure corresponding to the paging message is triggered for the CS domain, the MME determines from a Circuit Switched Fall Back (CSFB) response IE indicator in the Extended Service Request message whether the UE transmitting the Extended Service Request message accepts the request for the paging procedure triggered for the CS domain, and when the CSFB response IE indicator indicates that the UE accepts the request for the paging procedure triggered for the CS domain, the system performs the following operations:
1. the MME initiates a context setup procedure to establish an S1 bearer connection between the eNB and the MME and a radio bearer connection between the UE and the eNB and instructs the eNB to perform a Circuit Switched Fall Back (CSFB) procedure; and
2. the eNB initiates a PS handover according to the instruction of the MME to transfer the UE from a System Architecture Evolution (SAE) system to a GSM/EDGE Radio Access Network (GERAN) system or a Universal Terrestrial Radio Access Network (UTRAN) system; or the eNB performs a Network Assisted Cell Change (NACC) procedure according to the instruction of the MME.
When the UE is in the EMM-Connected status, the system is required to perform only the operation 2 among the foregoing operations to initiate a PS handover or perform an NACC procedure.
When then CSFB response IE indicator indicates that the UE refuses the request for the paging procedure triggered for the CS domain, the MME transmits an SGsAP-PAGING-REJECT message to a Mobile Switching Center (MSC) via an SGs interface between itself and a Visited Location Register (VLR), so that the VLR triggers a User determined UE busy (UDUB) procedure.
When the UE receives a paging message which indicates that the paging procedure corresponding to the paging message is triggered for the CS domain, if the UE is going to initiate an EMM procedure or a Service Request procedure, then there is no description in any existing protocol regarding how the UE will operate. Therefore there is a need of method for handling collision of a received paging message which indicates that the paging procedure corresponding to the paging message is triggered for the CS domain with an NAS procedure (including an EMM procedure and a Service Request procedure) initiated by a UE on its own initiative.